8 - Intelligence – Here and Elsewhere

Summary

With the exception of planets orbiting the most massive and luminous stars, planetary lifespans are measured in billions of years. Evolution on Earth has taken about four billion years so far, and probably has about another two or three billion to run, depending on when our ever-brightening Sun eventually boils away all our surface water. In the absence of evidence to the contrary, it’s probably a good idea to assume that evolution elsewhere takes billions of years too. It’s hard to imagine an evolutionary process in which intelligence is an early result rather than a late one. So, to look for intelligent alien life, we need to concentrate on planets that aren’t too young. Earlier, I suggested that good yardsticks for planetary age when looking for photosynthetic or intelligent life were at least two and four billion years, respectively. In general, we can imagine at least four stages in the life of a planet – no life at all, chemosynthetic life only, a stage characterized by a mixture of forms of energy acquisition including photosynthetic life, and a final stage that also includes intelligent life. In the present chapter, we’re concerned with the final one.